Glucose isotopomer analysis following addition of [U-13C]propionate yields relative 13C fluxes through anaplerosis, gluconeogenesis and the substrate cycle in perfused livers. We found that anaplerotic inflow was about 4 times the flux through citrate synthase, with a significant fraction (about 40%) being diverted into the substrate cycle leaving a net gluconeogenic output of about 2.4. In fasted rats, gluconeogenesis is equal to hepatic glucose output since there is no contribution from glycogenolysis. In perfused livers obtained from fasted rats, absolute gluconeogenic output can be easily estimated from direct measurements of glucose secretion into the perfusion fluid, allowing absolute flux measurements for the other pathways, including the substrate cycle, to be measured. However, measurement of hepatic glucose output in vivo requires a separate tracer measurement of glucose turnover. We're utiliziing the irreversible tracer, [1,6-13C2]glucose to measure glucoes turnover. There is no significant production of [1,6-13C2]glucose expected from hepatic metabolism of [U-13C]propionate, hence the labeling from [U-13C]propionate does not interfere with the glucose turnover measurement and the estimate (63mmol/min/kg) is consistent with published values. Correspondingly, there is no significant production of multiply-labeled glucose C2 isotopomers from the [1,6-13C2]glucose label, hence it does not interfere with the quantitation of the relative anaplerotic, gluconeogenic and substrate cycle fluxes from analysis of the glucose C2b multiplet ratios. The results indicate that of the 226 mmol/kg/min of anaplerotic units entering the oxaloacetate pool, about 55% entered the substrate + Cori cycles, with a gluconeogenic balance of about 125 mmol/kg/min of triose phosphate (63 mmol/kg/min of glucose). Given that the Cori cycle contributes only about 10-15% of the total carbon recycling between oxaloacetate and pyruvate it can be seen that there is substantial flux (~130-140 mmol/kg/min) through the substrate cycle in the fasted, anesthetized rat. (Service 10) REPORT PERIOD: (09/01/97-08/31/98)